DC level transition detecting circuit for sensor devices

Abstract

The DC sensor signal S, on which a low frequency waviness component S1 and a high frequency noise component S2, is obtained by amplitude detecting for a sinusoidal output signal from a touch signal probe. For generating a touch trigger signal TG at an sbrupt DC level transition of the DC sensor signal S, the signal S is input to a LPF 21 to output a signal (S0+S1), where S0 is a DC offset component, and S1 is a low frequency waviness component. The amplitude transforming circuit 22 multiplies the signal (S0+S1) by a coefficient k to generate a reference signal k(S0+S1). The comparator 23 compares the DC sensor signal S with the reference signal to output the touch trigger signal TG at the abrupt DC transition point of the DC sensor signal S.

What is claimed is: 
     
       1. A detecting circuit for detecting abrupt DC level transition in a DC sensor signal on which a waviness component and a noise component whose frequency is higher than the waviness component are superimposed, the abrupt DC level transition being generated in response to a to-be-detected physical change, comprising: a low-pass filter for inputting the DC sensor signal to remove the noise component and the abrupt DC level transition component in the DC sensor signal, thereby outputting an intermediate output signal including the waviness component;   amplitude transforming means for multiplying the intermediate output signal by a coefficient k to transform the amplitude of the intermediate output signal, thereby outputting a reference signal, the coefficient k being smaller than 1 in such a case that the abrupt DC level transition is in a direction of level decrease, the coefficient k being larger than 1 in such a case that the abrupt DC level transition is in a direction of level increase; and   comparing means for comparing the DC sensor signal with the reference signal output from the amplitude transforming means to extract the DC level transition component.   
     
     
       2. The detecting circuit according to claim 1, wherein: the coefficient k is set to be as close 1 as possible in the following inequality, in such a case that the abrupt DC level transition is in a direction of level decrease: ##EQU4## where, S0: DC component, S1: waviness component, S2: noise component.   
     
     
       3. The detecting circuit according to claim 1, wherein: the coefficient k is set to be as close as 1 in the following equality, in such a case that the abrupt DC level transition is in a direction of level decrease: ##EQU5## wherein, |S1|: positive maximum of the waviness component, -|S1|: negative maximum of the waviness component.   
     
     
       4. The detecting circuit according to claim 2, wherein: the coefficient k is set to be as close as 1 in the following equality, when DC component S0 is sufficiently larger than the waviness component |S1|: ##EQU6##   
     
     
       5. The detecting circuit according to claim 3, wherein: the coefficient k is set to be as close as 1 in the following equality, when DC component S0 is sufficiently larger than the waviness component |S1|: ##EQU7## 
     
     
       6. The detecting circuit according to claim 1, wherein: the DC sensor signal is an output of an amplitude detector for detecting an amplitude information in a sinusoidal signal output from an ultrasonic touch signal probe. 
     
     
       7. A touch sensor device, comprising: an ultrasonic touch signal probe on which a piezoelectric device is installed so as to be driven in a resonant state, thereby outputting a sinusoidal signal;   an amplitude detector for inputting the sinusoidal signal to detect the amplitude information of the sinusoidal signal, thereby outputting a DC sensor signal on which a waviness component and a noise component whose frequency is higher than the waviness component are superimposed, an abrupt DC level transition component being generated in the DC sensor signal when the touch signal probe is contacted with an object;   a low-pass filter for inputting the DC sensor signal to remove the noise component and the abrupt DC level transition component in the DC sensor signal, thereby outputting an intermediate output signal including the waviness component;   amplitude transforming means for multiplying the intermediate output signal by a coefficient k which is smaller than 1 to transform the amplitude of the intermediate output signal, thereby outputting a reference signal; and   comparing means for comparing the DC sensor signal with the reference signal output from the amplitude transforming means to extract the DC level transistor component.